1. Field of the Invention
This invention relates to pressure-sensitive adhesives that have good internal strength at high temperatures. The invention provides pressure-sensitive adhesives that can be coated without evolution of organic matter to provide tapes which are substantially odor-free and physiologically inert.
2. Description of the Related Art
When considering adhesive tapes, pressure-sensitive adhesive (PSA) tapes are the easiest to use, but for the most part, pressure-sensitive adhesives do not adhere well to nonpolar substrates. Another shortcoming is that most PSAs are unsuited for uses requiring good internal strength at elevated temperatures. For example, rubber-resin PSAs tend to soften and degrade when heated. PSAs based on styrene-containing block copolymers also do not retain good internal strength when heated, because styrene has a low T.sub.g and so softens at moderately elevated temperatures. Acrylate PSAs tend to give off toxic vapors at elevated temperatures. They typically contain monomeric materials which, even at ordinary room temperatures, exude odors that make acrylate PSA tapes generally unsuitable for medical uses. Polyisobutylene PSAs are often employed for medical uses because they are physiologically inert, but they tend to be deficient in internal strength.
PSAs are usually coated from solution or emulsion, thus releasing organic matter which would pollute the atmosphere unless recovered. Recovery apparatus is expensive and can be dangerous when used to recover inflammable solvents. Hence, it is preferable, when feasible, to hot-melt coat a PSA.
Of known PSAs, silicones best retain high internal strength at elevated temperatures, but known silicone PSAs must be coated from organic solvents. Typically, a metal catalyst is employed to initiate a reaction between gum and resin components, especially when good internal strength at elevated temperatures is required. Most effective are tin catalysts, the toxic nature of which prevents the resulting PSAs from being used in many important applications such as those involving food or medical needs. In spite of such problems and their high price, silicone PSA tapes are used where good internal strength at high temperatures is of utmost importance, e.g., as electrical insulating tapes and as masking tapes for use with paints to be baked at high temperatures.
PSAs can be based on .alpha.-olefin polymers. For example, U.S. Pat. No. 3,635,755 describes PSAs made from homopolymers of C.sub.6 to C.sub.11 .alpha.-olefins or from interpolymers of C.sub.2 to C.sub.16 .alpha.-olefins. These tapes are said to show substantially no irritation to skin and to have low shear adhesions that facilitate non-irritating removal from the human skin.
After noting that prior PSAs based on .alpha.-olefin polymers had very poor cohesive (internal) strength, U.S. Pat. No. 3,954,697 discloses that PSAs provided by copolymers of polypropylene and C.sub.6 to C.sub.10 .alpha.-olefins can be hot-melt coated at a melt temperature of at least 350.degree. F. (177.degree. C.) so that the copolymers exhibit no detectable crystallinity by either x-ray or DSC techniques. Nothing is said about cohesive strengths at elevated temperatures.
U.S. Pat. No. 4,288,358 discloses that a PSA adhesive based on .alpha.-olefin polymers can be hot-melt coated and can have good resistance to shear adhesion failure, i.e., good internal strength. This is accomplished by blending at least one C.sub.6 to C.sub.10 linear .alpha.-olefin polymer with a plasticizing oil and a tackifying resin. Nothing is said about internal strength at elevated temperatures.
Another publication of PSAs based on .alpha.-olefin copolymers wherein the monomers have up to 20 carbon atoms is U.S. Pat. No. 3,542,717.
U.S. Pat. No. 4,178,272 discloses that a hot-melt adhesive which provides strong T-peel and lap shear bonds can be made using .alpha.-olefin polymers. The hot-melt adhesive disclosed in this reference is a blend of poly(propylene-co-higher 1-olefin), tackifying resin, and crystalline polypropylene. The blend is not said to be tacky or provide a PSA. In Example 1, the bonds are made at 200.degree. C.
Another method of producing .alpha.-olefin polymers of good internal strength involves grafting silanes onto the polymers as in U.S. Pat. No. 4,759,992. The '992 patent relates to applying protective coatings to weatherable substrates such as wood, brick and concrete. The reference teaches moisture-curable silane-substituted .alpha.-olefin polymers that are of low molecular weight as evidenced by a vicosity average molecular weight of between about 500 and about 20,000.